Abstract

The existence of non-ribosomal rapidly-labelled ribonucleoprotein particles has been examined by isopycnic and sucrose gradient centrifugation in nuclear and cytoplasmic subcellular fractions of sheep thyroid tissue. These particles have been described in the saline extract (pH 8.0) of purified nuclei, in EDTA-treated polyribosomes and in postribosomal regions of deoxycholatetreated postmitochondrial supernatant. They have been identified as a special class of ribonucleoproteins rich in proteins by the following criteria: low buoyant densities in CsCl gradients (from 1.36 to 1.41 g/ml, with a predominance at 1.41), DNA-like base composition of their constitutive RNA, increase or decrease of their buoyant densities after trypsin or ribonuclease digestion. Although the specific nature of the constitutive protein(s) is not yet determined, it appears that both nuclei-associated and cytoplasmic (postribosomal and polyribosomal) rapidlylabelled ribonucleoproteins are complexes of messenger-like RNA and protein in a proportion of about 15 to 30% RNA, 85 to 70% protein by weight. RNA of cytoplasmic postribosomal ribonucleoprotein particles is heterogeneous and composed of species ranging from about 6 S to 22 S. It stimulates protein synthesis in the E. coli cell-free system whereas the corresponding ribonucleoproteins are poorly stimulatory. The relationships between these particles and their possible role in messenger RNA function are discussed.

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